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(11) | EP 0 384 900 A3 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Overheating protection triggering circuit having a narrow range of variation of the threshold |
| (57) An overheating protection triggering circuit for causing the switching-on of a guard
transistor when a certain preset threshold temperature is reached employs a circuit
for generating a reference current essentially proportional to a VBE/R1 where R1 is
a preset resistance and a circuit formed by two complementary current mirrors, the
first of which is emitter-degenerated by means of a preset resistance R2 in function
of a certain area ratio (i.e. δVBE) of the mirror. The circuit basically performs
a comparison between a reference current I1 = VBE/R1, strongly temperature dependent,
imposed by the generator on the mirror circuit and a current I3 = δVBE/R2 relatively
little dependent from temperature. With an increase of the temperature, the currents
flowing in the two branches of the mirror circuit equal each other at a presettable
threshold temperature thus causing the switching-on of a transistor. The temperature
threshold is preset during the design by choosing a certain area ratio and a certain
value for the resistance R2 of the first, emitter-degenerated current mirror and this
temperature threshold is relatively insensitive to the normal spread of parameters.
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